Objective assessment of the fusion frequency in functional electrical stimulation using the fast Fourier transform

In functional electrical stimulation (FES) the dynamics of tetanic muscle contractions is often described by the fusion frequency (FF), as determined by palpation: contractions elicited by stimulation frequencies above the FF appear smooth. To contribute to a more objective assessment of this important FES parameter, we have developed a dedicated signal analysis method based on fast Fourier transformation (FFT). The ripple to peak ratio (R(rpFFT)) - the relation between ripple amplitude and peak force value of a recorded tetanic muscle force in relation to the applied stimulation frequency - was determined automatically by analysing a 0.2-s interval in the steady state of a stimulation burst. The method was tested on simulated data and on force recordings from isolated tibialis anterior muscles of six rabbits. The results were compared to manual estimates. The robustness of the method was tested by adding noise and hum. Simulated noise at 100% of the ripple force increased R(rpFFT) by 4%. Hum at 20 Hz away from the stimulation frequency caused changes of less than 0.5%. The results of the automated analysis of recorded signals matched the manual estimates sufficiently well, especially for stimulation frequencies near or above FF. R(rpFFT) therefore seems suitable for automated, objective and robust assessment of the ripple and the FF of electrically stimulated muscle.